The importance of S-adenosylmethionine has been clearly established because of its role as the methyl donor in virtually all biological transmethylation reactions. Therefore, the overall objective of this program is concerned with the regulatory mechanisms involved in the microbial metabolism of S-adenosylmethionine, S-adenosylhomocysteine and the relationships of both compounds to sulfur metabolism and transmethylation reactions. This project will be concerned with the separation, purification and properties of S-adenosylmethionine lyase and 5'-methylthioadenosine nucleosidase of microbial cells. Wild type and various mutant strains of S. cerevisiae, C. utilis, C. albicans, A. aerogenes and E. coli will be used to study the induction, repression and feedback inhibition of S-adenosylmethionine lyase, 5'- methylthioadenosine nucleosidase, S-adenosylhomocysteine nucleosidase, homocysteine synthetase, homocysteine methyltransferase and S- adenosylmethionine synthetase. In addition the same microorganisms will be used for studies of the turnover of S-adenosylmethionine and S- adenosylhomocysteine and the relationship of both compounds to the biosynthesis of methionine. Experiments will be carried out to determine the utilization and metabolic fate of 5'-methylthioadenosine and S-adenosylhomocysteine, the two major metabolic derivatives of S- adenosylmethionine metabolism in microorganisms.